Power amplifier circuit

ABSTRACT

A power amplifier circuit includes a first amplifier transistor and a bias circuit. The first amplifier transistor amplifies a first signal and outputs a second signal. The bias circuit supplies a bias voltage or a bias current to the first amplifier transistor. The first amplifier transistor includes plural unit transistors disposed in a substantially rectangular region. The bias circuit includes first and second bias transistors and first and second voltage supply circuits. The first and second bias transistors respectively supply first and second bias voltages or first and second bias currents to the bases of unit transistors of first and second groups. The first and second voltage supply circuits respectively supply first and second voltages to the bases of the first and second bias transistors. The first and second voltages are decreased in accordance with a temperature increase. The second voltage supply circuit is disposed within the substantially rectangular region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to Japanese PatentApplication 2015-243586 filed Dec. 14, 2015, and to Japanese PatentApplication No. 2016-098075 filed May 16, 2016, the entire content ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a power amplifier circuit.

BACKGROUND

In mobile communication devices, such as cellular phones, a poweramplifier circuit is used for amplifying power of a radio frequency (RF)signal to be transmitted to a base station. As an amplifier element of apower amplifier circuit, a bipolar transistor, such as a hetero junctionbipolar transistor (HBT), is used.

When a bipolar transistor is driven with a constant base-emittervoltage, a collector current increases due to a temperature rise. Anincreased collector current increases power consumption, which raisesthe temperature of the transistor element and further increases thecollector current. This is called thermal runaway (positive feedback).It is thus desirable to suppress the occurrence of thermal runaway of abipolar transistor used in a power amplifier circuit. JapaneseUnexamined Patent Application Publication No. 2006-147665 discloses thefollowing configuration for suppressing the occurrence of thermalrunaway. In order to transfer a temperature change in a bipolartransistor to a temperature control element, thermal conduction wiringusing a high thermal conductive metal is provided so that the biasvoltage to be supplied to the bipolar transistor can be controlled.

SUMMARY

In the configuration disclosed in the above-described publication, theoccurrence of thermal runway is suppressed by the use of thermalconduction wiring that reduces the time for transferring a temperaturechange in the bipolar transistor to the temperature control element.This configuration increases the cost. Additionally, in some poweramplifier circuits, a bipolar transistor constituted by plural unittransistors (also called fingers) may be used. In such a bipolartransistor, the temperature distribution in the transistor element maybecome nonuniform. More specifically, the temperature in the centralportion of the transistor element becomes high, while the temperature inthe peripheral portion becomes low. Such a temperature distributionmakes the operation characteristics differ between the unit transistorsformed in the central portion of the transistor element and those formedin the peripheral portion, thereby decreasing the distortioncharacteristics of the bipolar transistor. The above-describedpublication does not disclose any measures to improve the uniformity ofthe temperature distribution in the transistor element of a bipolartransistor constituted by plural unit transistors.

The present disclosure has been made in view of the above-describedbackground. It is an object of the present disclosure to improve theuniformity of the temperature distribution in a bipolar transistorconstituted by plural unit transistors used in a power amplifiercircuit.

According to preferred embodiments of the present disclosure, there isprovided a power amplifier circuit including a first amplifiertransistor and a bias circuit. The first amplifier transistor amplifiesa first signal and outputs a second signal. The bias circuit supplies abias voltage or a bias current to the first amplifier transistor. Thefirst amplifier transistor includes a plurality of unit transistorsdisposed in a substantially rectangular region. The bias circuitincludes first and second bias transistors and first and second voltagesupply circuits. The first bias transistor supplies a first bias voltageor a first bias current to a base of a unit transistor of a first groupamong the plurality of unit transistors. The second bias transistorsupplies a second bias voltage or a second bias current to a base of aunit transistor of a second group among the plurality of unittransistors. The first voltage supply circuit supplies a first voltageto a base of the first bias transistor. The first voltage is decreasedin accordance with a temperature increase. The second voltage supplycircuit supplies a second voltage to a base of the second biastransistor. The second voltage is decreased in accordance with atemperature increase. The second voltage supply circuit is disposedwithin the substantially rectangular region.

According to preferred embodiments of the present disclosure, it ispossible to improve the uniformity of the temperature distribution in abipolar transistor constituted by plural unit transistors used in apower amplifier circuit.

Other features, elements, characteristics and advantages of the presentdisclosure will become more apparent from the following detaileddescription of preferred embodiments of the present disclosure withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of the configuration of a power amplifiercircuit according to an embodiment of the present disclosure.

FIG. 2 illustrates an example of the configurations of power amplifiersand bias circuits.

FIG. 3A illustrates an example of the layout of the power amplifiercircuit.

FIG. 3B illustrates another example of the layout of the power amplifiercircuit.

FIG. 3C illustrates still another example of the layout of the poweramplifier circuit.

FIG. 4 illustrates details of an example of the layout of the poweramplifiers and the bias circuits.

FIG. 5 illustrates an example of a cross section taken along line 5-5 inFIG. 4.

FIG. 6 illustrates an example of a cross section taken along line 6-6 inFIG. 4.

FIGS. 7 and 8 illustrate an example of simulation results of thetemperature distribution in the power amplifier circuit.

FIG. 9 illustrates another example of simulation results of thetemperature distribution in the power amplifier circuit.

FIG. 10 illustrates still another example of simulation results of thetemperature distribution in the power amplifier circuit.

FIG. 11 shows a graph illustrating an example of simulation resultsobtained from unit transistors arranged as shown in FIGS. 7, 9, and 10in accordance with the position of a voltage supply circuit.

FIG. 12 shows a graph illustrating an example of simulation resultsobtained by varying the distance (pitch) between the unit transistorsarranged shown in FIG. 9.

DETAILED DESCRIPTION

An embodiment of the present disclosure will be described below withreference to the accompanying drawings. FIG. 1 illustrates theconfiguration of a power amplifier circuit 100 according to anembodiment. The power amplifier circuit 100 is an integrated circuitused in a mobile communication device, such as a cellular phone, and foramplifying power of an RF signal to be transmitted to a base station.

As shown in FIG. 1, the power amplifier circuit 100 includes poweramplifiers 110, 120A, and 120B, bias circuits 130, 140A, and 140B,matching circuits (matching networks (MN)) 150 and 160, and inductors170 and 180.

The power amplifiers 110, 120A, and 120B form a two-stage amplifiercircuit. A power supply voltage Vcc is supplied to the power amplifier110 via the inductor 170 and is also supplied to the power amplifiers120A and 120B via the inductor 180. The power amplifier 110 amplifies anRF signal RFin1 (third signal) and outputs an amplified signal RFout1(first signal). The power amplifiers 120A and 120B amplify an RF signalRFin2 (RFout1) (first signal) and output an amplified signal RFout2(second signal). The power amplifiers 120A and 120B are connected inparallel with each other. The power amplifier circuit 100 may beoperated at a relatively low power level in a low power mode (LPM)(first power mode) and at a relatively high power level in a high powermode (HPM) (second power mode). The power amplifier 120A is turned ON inany one of the low power mode and the high power mode. The poweramplifier 120B is turned OFF in the low power mode and is turned ON inthe high power mode. In the low power mode, the power amplifier circuit100 amplifies an RF signal by using the power amplifiers 110 and 120A.In the high power mode, the power amplifier circuit 100 amplifies an RFsignal by using the power amplifiers 110, 120A, and 120B. The poweramplifiers 120A and 120B include a bipolar transistor (for example, anHBT) constituted by plural unit transistors (also called fingers). Thebipolar transistor includes, for example, sixteen unit transistors. Thepower amplifier 120A is constituted by four unit transistors, while thepower amplifier 120B is constituted by twelve unit transistors. Thenumber of unit transistors is not restricted to sixteen.

The bias circuits 130, 140A, and 140B supply a bias voltage or a biascurrent to the power amplifiers 110, 120A, and 120B, respectively. Abattery voltage Vbat is supplied to the bias circuits 130, 140A, and140B. The bias circuit 130 supplies a bias voltage or a bias current tothe power amplifier 110 on the basis of a bias control voltage Vbias1.Similarly, the bias circuits 140A and 140B respectively supply a biasvoltage or a bias current to the power amplifiers 120A and 120B on thebasis of bias control voltages Vbias2 and Vbias3. In the low power mode,the bias circuit 140B does not supply a bias voltage or a bias currentto the power amplifier 120B, which is thus turned OFF. The poweramplifier 120B may be turned OFF by another approach. For example, thesupply of a power supply voltage or a ground voltage to the poweramplifier 120B may be stopped.

The matching circuits 150 and 160 are provided for performing impedancematching between circuits. The matching circuits 150 and 160 areconstituted by, for example, inductors and capacitors.

FIG. 2 illustrates an example of the configurations of the poweramplifiers 120A and 120B and the bias circuits 140A and 140B.

The power amplifiers 120A and 120B include a bipolar transistor 200(first amplifier transistor) constituted by plural units transistors.The bipolar transistor 200 is constituted by a first group of plural(for example, four) unit transistors and a second group of plural (forexample, twelve) unit transistors. The power amplifier 120A includesunit transistors 210A of the first group, resistors 211A, and capacitors212A. The power amplifier 120B includes unit transistors 210B of thesecond group, resistors 211B, and capacitors 212B.

In the unit transistor 210A, the collector receives the power supplyvoltage Vcc via the inductor 180, the base receives the RF signal RFin2via the capacitor 212A, and the emitter is grounded. A bias voltage or abias current is supplied to the base of the unit transistor 210A via theresistor 211A. In the unit transistor 210B, the collector receives thepower supply voltage Vcc via the inductor 180, the base receives the RFsignal RFin2 via the capacitor 212B, and the emitter is grounded. A biasvoltage or a bias current is supplied to the base of the unit transistor210B via the resistor 211B. With this configuration, the amplifiedsignal RFout2 is output from the collector of the bipolar transistor200.

The bias circuit 140A includes a bipolar transistor 220A (for example,an HBT), a voltage supply circuit 221A, a capacitor 222A, and a resistor223A.

In the bipolar transistor 220A (first bias transistor), the collectorreceives the battery voltage Vbat, the base receives a voltage (firstvoltage) from the voltage supply circuit 221A, and the emitter suppliesa bias voltage (first bias voltage) or a bias current (first biascurrent) to the base of the unit transistor 210A via the resistor 211A.

The voltage supply circuit 221A (first voltage supply circuit) controlsthe base voltage of the bipolar transistor 220A on the basis of the biascontrol voltage Vbias2. The voltage supply circuit 221A includes diodes230A and 231A (first and second diodes). The diodes 230A and 231A areconnected in series with each other. The anode of the diode 230A isconnected to the base of the bipolar transistor 220A, and the cathode ofthe diode 231A is grounded. The capacitor 222A is connected in parallelwith the diodes 230A and 231A. The bias control voltage Vbias2 issupplied to the anode of the diode 230A via the resistor 223A. With theconfiguration of the voltage supply circuit 221A, a voltage (firstvoltage) corresponding to a forward voltage of the diodes 230A and 231Ais generated in the anode of the diode 230A, and this voltage issupplied to the base of the bipolar transistor 220A. Due to thecharacteristics of the forward voltage of the diodes 230A and 231A, thevoltage generated in the anode of the diode 230A decreases in accordancewith a temperature rise. The capacitor 222A is provided for stabilizingthe voltage supplied from the voltage supply circuit 221A. The diodes230A and 231A of the voltage supply circuit 221A will also be indicatedby D1 and D2, respectively. The diodes 230A and 231A may be eachconstituted by a diode-connected bipolar transistor. Although in theexample shown in FIG. 2 the voltage supply circuit 221A is constitutedby diodes, the elements forming the voltage supply circuit 221A are notrestricted to diodes.

The bias circuit 140B includes a bipolar transistor 220B (for example,an HBT), a voltage supply circuit 221B, a capacitor 222B, and a resistor223B.

In the bipolar transistor 220B (second bias transistor), the collectorreceives the battery voltage Vbat, the base receives a voltage (secondvoltage) from the voltage supply circuit 221B, and the emitter suppliesa bias voltage (second bias voltage) or a bias current (second biascurrent) to the base of the unit transistor 210B via the resistor 211B.

The voltage supply circuit 221B (second voltage supply circuit) controlsthe base voltage of the bipolar transistor 220B on the basis of the biascontrol voltage Vbias3. The voltage supply circuit 221B includes diodes230B and 231B (third and fourth diodes). The diodes 230B and 231B areconnected in series with each other. The anode of the diode 230B isconnected to the base of the bipolar transistor 220B, and the cathode ofthe diode 231B is grounded. The capacitor 222B is connected in parallelwith the diodes 230B and 231B. The bias control voltage Vbias3 issupplied to the anode of the diode 230B via the resistor 223B. With theconfiguration of the voltage supply circuit 221B, a voltage (secondvoltage) corresponding to a forward voltage of the diodes 230B and 231Bis generated in the anode of the diode 230B, and this voltage issupplied to the base of the bipolar transistor 220B. Due to thecharacteristics of the forward voltage of the diodes 230B and 231B, thevoltage generated in the anode of the diode 230B decreases in accordancewith a temperature rise. The capacitor 222B is provided for stabilizingthe voltage supplied from the voltage supply circuit 221B. The diodes230B and 231B of the voltage supply circuit 221B will also be indicatedby D1 and D2, respectively. The diodes 230B and 231B may be eachconstituted by a diode-connected bipolar transistor. Although in theexample shown in FIG. 2 the voltage supply circuit 221B is constitutedby diodes, the elements forming the voltage supply circuit 221B are notrestricted to diodes.

The configuration of the power amplifier 110 is similar to that of thepower amplifiers 120A and 120B shown in FIG. 2, and the configuration ofthe bias circuit 130 is similar to that of the bias circuits 140A and140B shown in FIG. 2. That is, as well as the power amplifiers 120A and120B, the power amplifier 110 includes a bipolar transistor (secondamplifier transistor) as the amplifier element.

FIG. 3A illustrates an example of the layout of the power amplifiercircuit 100. The layout shown in FIG. 3A does not illustrate theconfiguration of all the elements of the power amplifier circuit 100,but only schematically illustrates the configuration of the poweramplifier circuit 100.

As shown in FIG. 3A, the voltage supply circuit 221A forming the biascircuit 140A is disposed outside a substantially rectangular regionwhere the bipolar transistor 200 is formed. In contrast, the voltagesupply circuit 221B forming the bias circuit 140B is disposed withinthis substantially rectangular region. The power amplifier 120A isdisposed in a region (first sub-region) which does not include thecentral portion of the substantially rectangular region where thebipolar transistor 200 is formed. The central portion of thesubstantially rectangular region is an intersection of two diagonallines in the substantially rectangular region. In contrast, the poweramplifier 120B is disposed in a region (second sub-region) whichincludes the central portion of the substantially rectangular regionwhere the bipolar transistor 200 is formed. The power amplifier 120A isdisposed in a region which does not include the central portion of thesubstantially rectangular region which means that part of the bipolartransistor 200 forming the power amplifier 120A is not disposed at thecentral portion of the substantially rectangular region. The poweramplifier 120B is disposed in a region which includes the centralportion of the substantially rectangular region which means that part ofthe bipolar transistor 200 forming the power amplifier 120B is disposedat the central portion of the substantially rectangular region. Detailsof the positions at which the power amplifiers 120A and 120B aredisposed will be discussed later.

The temperature of the bipolar transistor 200 rises in accordance withits operation. The temperature rise is noticeable particularly when thebipolar transistor 200 operates in the high power mode. This temperaturerise increases the collector current in the bipolar transistor 200, andthe increased collector current raises the temperature of the bipolartransistor 200 again. This may cause the occurrence of thermal runaway.In the power amplifier circuit 100, the voltage supply circuits 221A and221B contribute to suppressing the occurrence of thermal runaway.

A rise in the temperature of the bipolar transistor 200 increases thetemperature of the voltage supply circuit 221B. This decreases theforward voltage of the diodes 230B and 231B and further decreases thevoltage supplied to the base of the bipolar transistor 220B. Thisdecreases the bias voltage or the bias current supplied to the poweramplifier 120B. As a result, a temperature rise in the bipolartransistor 200 is suppressed. Decreasing of the forward voltage of thediodes 230B and 231B by heat is called thermal coupling between theamplifier and the voltage supply circuit.

Regarding the temperature distribution in the bipolar transistor 200,without the control performed by the voltage supply circuits 221A and221B, the temperature at and around the center of the transistor elementbecomes high, while the temperature at and near the periphery thereofbecomes low. That is, in the bipolar transistor 200, the temperature ofthe power amplifier 120A becomes relatively low. In this embodiment, thevoltage supply circuit 221B is disposed within the substantiallyrectangular region where the bipolar transistor 200 is formed, while thevoltage supply circuit 221A is disposed outside this substantiallyrectangular region. According to this layout, the temperature of thevoltage supply circuit 221A becomes lower than that of the voltagesupply circuit 221B. Consequently, the bias voltage or the bias currentsupplied to the power amplifier 120A is not decreased as much as that tothe power amplifier 120B. This suppresses a temperature decrease in theregion where the power amplifier 120A is disposed. As a result, theuniformity of the temperature distribution in the overall bipolartransistor 200 can be improved.

It is preferable that the voltage supply circuit 221A be disposed in aregion adjacent to the substantially rectangular region where thebipolar transistor 200 is formed. In the layout shown in FIG. 3A, forexample, the voltage supply circuit 221A is formed between the outerperiphery of the substantially rectangular region where the bipolartransistor 200 is formed and the bipolar transistor 220A of the biascircuit 140A. The temperature of the region adjacent to thesubstantially rectangular region where the bipolar transistor 200 isformed is increased in accordance with a temperature rise in the bipolartransistor 200, though it is lower than that of the substantiallyrectangular region. The temperature of the voltage supply circuit 221Ais increased accordingly, which further decreases the bias voltage orthe bias current supplied to the power amplifier 120A. As a result, theoccurrence of thermal runaway can be suppressed.

The region where the voltage supply circuit 221A is disposed is notrestricted to that shown in FIG. 3A. The voltage supply circuit 221A maybe disposed at any region adjacent to the substantially rectangularregion where the bipolar transistor 200 is formed. As shown in FIG. 3B,for example, the voltage supply circuit 221A may be disposed in a region300 between the outer periphery of the substantially rectangular regionwhere the bipolar transistor 200 is formed and another element such asthe power amplifier 110. As shown in FIG. 3C, for example, the voltagesupply circuit 221A may be disposed in a region 320 between the outerperiphery of the substantially rectangular region where the bipolartransistor 200 is formed and wire-bonding terminals 310 through 313.Alternatively, the voltage supply circuit 221A may not necessarily bedisposed in a region adjacent to the substantially rectangular regionwhere the bipolar transistor 200 is formed. For example, another elementmay be disposed between the substantially rectangular region where thebipolar transistor 200 is formed and the voltage supply circuit 221A.

FIG. 4 illustrates details of an example of the layout of the poweramplifiers 120A and 120B and the bias circuits 140A and 140B.

In FIG. 4, sixteen unit transistors (fingers) F1 through F16 forming thebipolar transistor 200 are shown. The sixteen unit transistors arealigned in two rows (F1 through F8 and F9 through F16). The poweramplifier 120A includes four unit transistors F1, F2, F9, and F10. Thepower amplifier 120B includes twelve unit transistors F3 through F8 andF11 through F16. The unit transistors F1, F2, F9, and F10 are disposedin a region (first sub-region) which does not include the centralportion of the substantially rectangular region where the bipolartransistor 200 is formed. The unit transistors F3 through F8 and F11through F16 are disposed in a region (second sub-region) which includesthe central portion of this substantially rectangular region.

The RF signal RFin2 is supplied to the base of each unit transistor viaRF input wiring 400. A bias voltage or a bias current is supplied to thebase of each of the unit transistors F1, F2, F9, and F10 of the poweramplifier 120A from the bipolar transistor 220A via wiring 410. A biasvoltage or a bias current is supplied to the base of each of the unittransistors F3 through F8 and F11 through F16 of the power amplifier120B from the bipolar transistor 220B via wiring 420. The collector ofeach unit transistor is connected to collector wiring 430. The emitterof each unit transistor is connected to emitter wiring 440 and isgrounded through a via-hole 450. The number of unit transistors is notrestricted to sixteen, and the number of rows of the unit transistors isnot restricted to two.

As described above, the voltage supply circuit 221A (diodes 230A and231A) is disposed outside the substantially rectangular region where thebipolar transistor 200 is formed. More specifically, the voltage supplycircuit 221A (diodes 230A and 231A) is disposed at a position at whichit is separated from the outer periphery of this substantiallyrectangular region by a distance d. The voltage supply circuit 221B(diodes 230B and 231B) is disposed within this substantially rectangularregion. According to this layout, it is possible to improve theuniformity of the temperature distribution in the bipolar transistor200, as discussed above.

As shown in FIG. 4, the unit transistors F1 through F8 on the side onwhich the voltage supply circuit 221A is not disposed may be arrangedsubstantially symmetrically with the unit transistors F9 through F16 onthe side on which the voltage supply circuit 221A is disposed. With thisarrangement, the unit transistors F1 through F16, which are heatsources, are disposed substantially symmetrically with respect to thecenter of the substantially rectangular region where the bipolartransistor 200 is formed. This further contributes to an improvement inthe uniformity of the temperature distribution in the bipolar transistor200. Unit transistors disposed in three or more rows may be arranged ina similar manner.

Another element may be disposed in a space within the substantiallyrectangular region where the bipolar transistor 200 is formed. Forexample, a protective element may be disposed between the unittransistors F4 and F5. Forming of another element within thesubstantially rectangular region reduces the chip size of the poweramplifier circuit 100.

FIG. 5 illustrates an example of a cross section of a unit transistortaken along line 5-5 in FIG. 4. The unit transistor includes asub-collector 500, a collector 510, collector electrodes 511, a base520, base electrodes 521, an emitter 530, and an emitter electrode 531.

The sub-collector 500 is formed on, for example, a gallium arsenide(GaAs) substrate 540. The collector 510 and the collector electrodes 511are formed on the sub-collector 500. The base 520 is formed on thecollector 510. The base electrodes 521 are formed on the base 520.Collector wiring 550 and the collector wiring 430 shown in FIG. 4 arelaid on the collector electrodes 511. The emitter electrode 531 isformed on the emitter 530. The emitter wiring 440 shown in FIG. 4 islaid on the emitter electrode 531.

FIG. 6 illustrates an example of a cross section taken along line 6-6 inFIG. 4. The emitter wiring 440 is formed on the front surface of thesubstrate 540. An insulating resin film 600 is formed on the emitterwiring 440. The collector wiring 430 is formed on the insulating resinfilm 600. The via-hole 450 is formed from the back surface of thesubstrate 540 until the emitter wiring 440. Wiring 610 to be connectedto a ground is formed in the via-hole 450.

FIGS. 7 and 8 illustrate an example of the simulation results of thetemperature distribution in the power amplifier circuit 100.

FIG. 7 illustrates the temperatures of unit transistors. As shown inFIG. 7, sixteen unit transistors (F1 through F16) are arranged in tworows (F1 through F8 and F9 through F16). Among the sixteen unittransistors F1 through F16, the unit transistors F1, F2, F9, and F10 areused for the power amplifier 120A. The voltage supply circuit 221A(diodes 230A and 231A) is disposed at a position at which it isseparated by about 40 μm from the outer periphery of the substantiallyrectangular region where the bipolar transistor 200 is formed. Thevoltage supply circuit 221B (diodes 230B and 231B) is disposed near thecentral portion of the substantially rectangular region (between theunit transistors F12 and F13).

In FIG. 7, the temperatures of the unit transistors when the poweramplifier circuit 100 is operated in the high power mode (at a roomtemperature of about 25 degrees) are shown. FIG. 7 shows that thetemperatures of the unit transistors F1, F2, F9, and F10 are roughly thesame as those of the unit transistors disposed near the central portionof the substantially rectangular region (for example, unit transistorsF4, F5, F12, and F13).

FIG. 8 shows a graph representing the thermal resistance of the unittransistors. The horizontal axis indicates the position of the unittransistor, while the vertical axis indicates the thermal resistance (°C./W). In FIG. 8, the line chart indicated by “separated” represents thesimulation results of the power amplifier circuit 100. In FIG. 8, theline chart indicated by “not separated” represents the simulationresults of a comparative example in which, as well as the voltage supplycircuit 221B (diodes 230B and 231B), the voltage supply circuit 221A(diodes 230A and 231A) is disposed within the substantially rectangularregion where the bipolar transistor 200 is formed. FIG. 8 shows that thevariations in the thermal resistance of the unit transistors F1 throughF16 of the power amplifier circuit 100 are smaller than those of thecomparative example.

FIG. 9 illustrates another example of the simulation results of thetemperature distribution in the power amplifier circuit 100. In FIG. 9,the temperatures of the unit transistors when the power amplifiercircuit 100 is operated in the high power mode (at a room temperature ofabout 25 degrees) are shown. In FIG. 9, sixteen unit transistors F1through F16 are arranged in one row. Among the sixteen unit transistorsF1 through F16, the unit transistors F1, F2, F15, and F16 are used forthe power amplifier 120A. The voltage supply circuit 221A (diodes 230Aand 231A) is disposed at a position at which it is separated by about150 μm from the outer periphery of the substantially rectangular regionwhere the bipolar transistor 200 is formed. The voltage supply circuit221B (diodes 230B and 231B) is disposed near the central portion of thesubstantially rectangular region (between the unit transistors F8 andF9). In this example, as well as in the example shown in FIG. 7, thetemperatures of the unit transistors F1, F2, F15, and F16 are roughlythe same as those of the unit transistors disposed near the centralportion of the substantially rectangular region (for example, unittransistors F8 and F9).

FIG. 10 illustrates another example of the simulation results of thetemperature distribution in the power amplifier circuit 100. In FIG. 10,the temperatures of the unit transistors when the power amplifiercircuit 100 is operated in the high power mode (at a room temperature ofabout 25 degrees) are shown. In FIG. 10, sixteen unit transistors (F1through F16) are arranged in four rows (F1 through F4, F5 through F8, F9through F12, and F13 through F16). Among the sixteen unit transistors F1through F16, the unit transistors F1, F2, F13, and F14 are used for thepower amplifier 120A. The voltage supply circuit 221A (diodes 230A and231A) is disposed at a position at which it is separated by about 60 μmfrom the outer periphery of the substantially rectangular region wherethe bipolar transistor 200 is formed. The voltage supply circuit 221B(diodes 230B and 231B) is disposed near the central portion of thesubstantially rectangular region (between the unit transistors F6 andF7). In this example, as well as in the examples shown in FIGS. 7 and 9,the temperatures of the unit transistors F1, F2, F13, and F14 areroughly the same as those of the unit transistors disposed near thecentral portion of the substantially rectangular region (for example,unit transistors F6, F7, F10, and F11).

FIG. 11 shows a graph illustrating an example of the simulation resultsobtained from the unit transistors arranged in two rows, those arrangedin one row, and those arranged in four rows shown in FIGS. 7, 9, and 10,respectively, in accordance with the position of the voltage supplycircuit 221A (diodes 230A and 231A).

In FIG. 11, the horizontal axis indicates the ratio (%) of thetemperature (Tave (D1,D2)) at the position at which the voltage supplycircuit 221A (diodes 230A and 231A) is disposed to the maximumtemperature Tmax in the substantially rectangular region where thebipolar transistor 200 is formed. The vertical axis indicates the ratio(%) of the standard deviation (σ) of the thermal resistance of the unittransistors to the average (ave) of the thermal resistance of the unittransistors. In FIG. 11, data representing the temperature ratio around90% on the horizontal axis indicates the simulation results of acomparative example (“not separated”), as in FIG. 8.

The simulation results in FIG. 11 show that the variations in thethermal resistance of the unit transistors arranged in two rows, thosein one row, and those in four rows are all smaller than those of thecomparative example, that is, the uniformity of the temperaturedistribution in the unit transistors is improved. In the range of thetemperature ratio of about 60 to 75% on the horizontal axis, thevariations in the thermal resistance are particularly small.

FIG. 12 shows a graph illustrating an example of the simulation resultsobtained by varying the distance (pitch) between the unit transistorsarranged in one row shown in FIG. 9. In FIG. 12, the horizontal axisindicates the temperature ratio and the vertical axis indicates theratio of the standard deviation, as in FIG. 11. In FIG. 12, thesimulation results obtained by varying the pitch between the unittransistors to about 30 μm, 35 μm, and 40 μm are shown. Regardless ofthe pitch between the unit transistors, the variations in the thermalresistance are particularly small in the range of the temperature ratioof about 60 to 75% on the horizontal axis.

The exemplary embodiment has been discussed. In the power amplifiercircuit 100, the unit transistors 210A of the first group forming thepower amplifier 120A are disposed in a region (first sub-region) whichdoes not include the central portion of the substantially rectangularregion where the bipolar transistor 200 is formed. In contrast, the unittransistors 210B of the second group forming the power amplifier 120Bare disposed in a region (second sub-region) which includes the centralportion of the substantially rectangular region where the bipolartransistor 200 is formed. The voltage supply circuit 221A, whichcontrols a bias voltage or a bias current to be supplied to the poweramplifier 120A, is disposed outside the substantially rectangular regionwhere the bipolar transistor 200 is formed. In contrast, the voltagesupply circuit 221B, which controls a bias voltage or a bias current tobe supplied to the power amplifier 120B, is disposed within thissubstantially rectangular region.

According to this layout, the temperature of the voltage supply circuit221A becomes lower than that of the voltage supply circuit 221B.Consequently, the bias voltage or the bias current supplied to the poweramplifier 120A is not decreased as much as that to the power amplifier120B. This suppresses a temperature decrease in the region where thepower amplifier 120A is disposed (the region which does not include thecentral portion of the substantially rectangular region where thebipolar transistor 200 is formed). As a result, the uniformity of thetemperature distribution in the bipolar transistor 200 can be improved.

In the power amplifier circuit 100, the voltage supply circuit 221B isdisposed in a region (second sub-region) which includes the centralportion of the substantially rectangular region where the bipolartransistor 200 is formed. The temperature of the bipolar transistor 200,in particular, the central portion, is likely to become high. Forming ofthe voltage supply circuit 221B near the central portion of thesubstantially rectangular region enhances the effect of suppressing theoccurrence of thermal runaway in the bipolar transistor 200.

In the power amplifier circuit 100, the number of unit transistors(fingers) forming the power amplifier 120A is smaller than that of thepower amplifier 120B. Thus, fewer unit transistors (fingers) benefitfrom the effect of suppressing a temperature decrease. To put it anotherway, it is easy to maintain the effect of suppressing the occurrence ofthermal runaway in the overall bipolar transistor 200.

In the power amplifier circuit 100, the voltage supply circuit 221A isdisposed in a region adjacent to the substantially rectangular regionwhere the bipolar transistor 200 is formed. The temperature of theregion adjacent to this substantially rectangular region is increased inaccordance with a temperature rise in the bipolar transistor 200, thoughit is lower than that of the substantially rectangular region. Thetemperature of the voltage supply circuit 221A is increased accordingly,which further decreases the bias voltage or the bias current supplied tothe power amplifier 120A. As a result, the occurrence of thermal runawaycan be suppressed.

As shown in FIG. 3A, for example, the voltage supply circuit 221A may bedisposed between the outer periphery of the substantially rectangularregion where the bipolar transistor 200 is formed and the bipolartransistor 220A of the bias circuit 140A.

As shown in FIG. 3B, for example, the voltage supply circuit 221A may bedisposed in the region 300 between the outer periphery of thesubstantially rectangular region where the bipolar transistor 200 isformed and another element such as the power amplifier 110.

As shown in FIG. 3C, for example, the voltage supply circuit 221A may bedisposed in the region 320 between the outer periphery of thesubstantially rectangular region where the bipolar transistor 200 isformed and wire-bonding terminals 310 through 313.

As shown in FIGS. 11 and 12, the voltage supply circuit 221A may bedisposed at a position at which the temperature of the voltage supplycircuit 221A will be about 60 to 75% of the maximum temperature in thesubstantially rectangular region where the bipolar transistor 200 isformed. This further increases the uniformity of the temperaturedistribution.

In the power amplifier circuit 100, the voltage supply circuit 221A maybe constituted by the series-connected diodes 230A and 231A. Similarly,the voltage supply circuit 221B may be constituted by theseries-connected diodes 230B and 231B. With this configuration, theoccurrence of thermal runaway can be suppressed without the need to usea high resistance resistor.

The above-described preferred embodiment is provided for facilitatingthe understanding of the disclosure, but is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Modifications and/or improvements may be made to the disclosure withoutdeparting from the scope and spirit of the disclosure, and equivalentsof the disclosure are also encompassed in the disclosure. That is,suitable design changes made to the preferred embodiment by thoseskilled in the art are also encompassed in the disclosure within thescope and spirit of the disclosure. For example, the elements of thepreferred embodiment and the positions, materials, conditions,configurations, and sizes thereof are not restricted to those describedin the embodiment and may be changed in an appropriate manner. Theelements of the preferred embodiment may be combined within atechnically possible range, and configurations obtained by combining theelements of the embodiment are also encompassed in the disclosure withinthe scope and spirit of the disclosure.

While preferred embodiments of the disclosure have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the disclosure. The scope of the disclosure, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A power amplifier circuit comprising: a firstamplifier transistor that amplifies a first signal inputted into aninput terminal and outputs a second signal; and a bias circuit thatsupplies a bias voltage or a bias current to the first amplifiertransistor, the first amplifier transistor including a plurality of unittransistors, the bias circuit including a bias transistor that suppliesa bias voltage or a bias current to a base of a unit transistor amongthe plurality of unit transistors, and a voltage supply circuit thatsupplies a voltage to a base of the bias transistor, the voltage beingdecreased in accordance with a temperature increase, wherein the voltagesupply circuit includes two diodes: the one of the two diodes has ananode connected to the base of the bias transistor and a cathode; andthe other diode has an anode connected to the cathode of the diode whichanode is connected to the base of the bias transistor and a cathodewhich is grounded, and the other diode is disposed within an area formedby the outermost unit transistors among the plurality of unittransistors.
 2. The power amplifier circuit according to claim 1,wherein the unit transistors are heterojunction bipolar transistors. 3.The power amplifier circuit according to claim 2, wherein the biastransistor is a heterojunction bipolar transistor.
 4. The poweramplifier circuit according to claim 3, wherein each unit transistor hasa capacitor connected between the input terminal and the base of eachunit transistor.
 5. The power amplifier circuit according to claim 4,wherein each unit transistor has a resistor connected between theemitter of the bias transistor and the base of each unit transistor. 6.The power amplifier circuit according to claim 5, wherein each of thetwo diodes is made of a heterojunction bipolar transistor with the baseand collector being shorted electrically.
 7. The power amplifier circuitaccording to claim 6, wherein the area formed by the outermost unittransistors among the plurality of unit transistors is a substantiallyrectangular region.
 8. A power amplifier circuit comprising: a firstamplifier transistor that amplifies a first signal inputted into aninput terminal and outputs a second signal; and a bias circuit thatsupplies a bias voltage or a bias current to the first amplifiertransistor, the first amplifier transistor including a plurality of unittransistors of a first group and a plurality of unit transistors of asecond group, the bias circuit including a bias transistor that suppliesa bias voltage or a bias current to a base of a unit transistors of thesecond group among the plurality of unit transistors, and a voltagesupply circuit that supplies a voltage to a base of the bias transistor,the voltage being decreased in accordance with a temperature increase,wherein the voltage supply circuit includes two diodes: the one of thetwo diodes has an anode connected to the base of the bias transistor anda cathode; and the other diode has an anode connected to the cathode ofthe diode which anode is connected to the base of the bias transistorand a cathode which is grounded, and the other diode is disposed withinan area formed by the outermost unit transistors of the second groupamong the plurality of unit transistors.
 9. The power amplifier circuitaccording to claim 8, wherein the unit transistors of the first groupand the second group are heterojunction bipolar transistors.
 10. Thepower amplifier circuit according to claim 9, wherein the biastransistor is a heterojunction bipolar transistor.
 11. The poweramplifier circuit according to claim 10, wherein each unit transistor ofthe first group and the second group has a capacitor connected betweenthe input terminal and the base of each unit transistor of the firstgroup and the second group.
 12. The power amplifier circuit according toclaim 11, wherein each unit transistor of the second group has aresistor connected between the emitter of the bias transistor and thebase of each unit transistor of the second group.
 13. The poweramplifier circuit according to claim 12, wherein each of the two diodesis made of a heterojunction bipolar transistor with the base andcollector being shorted electrically.
 14. The power amplifier circuitaccording to claim 13, wherein an area formed by the outermost unittransistors of the second group among the plurality of unit transistorsis a substantially rectangular region.
 15. A power amplifier circuitcomprising: a first amplifier transistor that amplifies a first signalinputted into an input terminal and outputs a second signal; and a biascircuit that supplies a bias voltage or a bias current to the firstamplifier transistor, the first amplifier transistor including aplurality of unit transistors of a first group and a plurality of unittransistors of a second group, the bias circuit including a first biastransistor that supplies a first bias voltage or a first bias current toa base of a unit transistors of the first group among the plurality ofunit transistors, a second bias transistor that supplies a second biasvoltage or a second bias current to a base of a unit transistors of thesecond group among the plurality of unit transistors, a first voltagesupply circuit that supplies a first voltage to a base of the first biastransistor, the first voltage being decreased in accordance with atemperature increase, and a second voltage supply circuit that suppliesa second voltage to a base of the second bias transistor, the secondvoltage being decreased in accordance with a temperature increase,wherein the second voltage supply circuit includes two diodes: the oneof the two diodes has an anode connected to the base of the second biastransistor and a cathode; and the other diode has an anode connected tothe cathode of the diode which anode is connected to the base of thesecond bias transistor and a cathode which is grounded, and the otherdiode is disposed within an area formed by the outermost unittransistors of the second group among the plurality of unit transistors.16. The power amplifier circuit according to claim 15, wherein the unittransistors of the first group and the second group are heterojunctionbipolar transistors.
 17. The power amplifier circuit according to claim16, wherein the first bias transistor and the second bias transistor areheterojunction bipolar transistors.
 18. The power amplifier circuitaccording to claim 17, wherein each unit transistor of the first groupand the second group has a capacitor connected between the inputterminal and the base of each unit transistor of the first group and thesecond group.
 19. The power amplifier circuit according to claim 18,wherein each unit transistor of the first group has a resistor connectedbetween the emitter of the first bias transistor and the base of eachunit transistor of the first group, and each unit transistor of thesecond group has a resistor connected between the emitter of the secondbias transistor and the base of each unit transistor of the secondgroup.
 20. The power amplifier circuit according to claim 19, whereineach of the two diodes is made of a heterojunction bipolar transistorwith the base and collector being shorted electrically.
 21. The poweramplifier circuit according to claim 20, wherein an area formed by theoutermost unit transistors of the second group among the plurality ofunit transistors is a substantially rectangular region.